1. Why Are Swallow Nests Built on Human Buildings?
Swallow nests are found on houses, warehouses, garages, barns, shops, gas stations, office buildings, and pedestrian bridges. In Japan, there are no reports of nests built on structures other than buildings.[1]
One theory is that swallows use humans to protect their chicks from predators like crows and snakes.[2] This is considered a hypothesis, not a proven fact for every location. The Wild Bird Society of Japan notes that swallows have long been appreciated as beneficial birds that eat crop pests, and nests on houses are said to bring good luck.[3]
2. Where Do Swallows Get the Mud for Their Nests?
A swallow’s nest is mainly made of mud, reinforced with grass around the egg-laying area. It looks like a half-cut bowl, with the flat side attached to a wall.[1]
Swallows live in areas with fields and farms, from plains to mountain regions. They are especially common in rural spots where rice paddies touch forests.[1] This makes sense because these places offer both mud and insects. A report from Machida City states that in cities paved with asphalt, it is harder to find mud for nests, and insect numbers may be insufficient.[2]
A national survey by the Wild Bird Society of Japan found that the average number of chicks leaving a nest was about 4. In urban areas, it was 3.8, while in other areas, it was 4.2. The difference of 0.4 chicks means that over 10 nests, there is a difference of about 4 chicks.[3] Even if a nest is built, raising chicks requires many insects.
3. How Do Swallows Catch Insects to Feed Their Chicks?
Swallows rarely land on the ground. They catch insects like bees, flies, horseflies, and dragonflies in mid-air. They also drink water by skimming the surface while flying.[4]
They bring flying insects such as flies, horseflies, bees, winged ants, and mayflies to their chicks. During the rainy season, they may also bring katydids larger than 2 cm (about 0.8 inches) or moth caterpillars about 3 cm (about 1.2 inches) long.[1]
It is said that a swallow catches close to several thousand insects in one day.[2] This number comes from municipal reports and should be seen as an estimate. From hatching to leaving the nest, the period is 17 to 23 days, or about 3 weeks.[1] After leaving the nest, where do the young swallows sleep?
4. Where Do Young Swallows Sleep After Leaving the Nest?
Young swallows stay near the nest for a few days, getting food from their parents. They separate from their parents after about 2 weeks. Then, they begin to spend nights in groups at water edges, such as in reed beds.[4]
These group roosts can contain thousands to tens of thousands of swallows. Near Tokyo, they start gathering in mid-June and reach their maximum number from late July to early August.[4]
In July and August, swallows that have finished raising chicks or have just left the nest gather in reed beds along riverbanks. The sight of them dancing in the sky at dusk and descending together to sleep is called "roosting entry" and fascinates many people.[5] The Wild Bird Society of Japan publishes a map of roosts nationwide and a pamphlet called "Good Night, Swallows." You can use the map to see if there is a roost nearby. This shows that the place for raising young and the place for sleeping are different.
5. Where Do Swallows Go When Autumn Comes?
In the Kanto region, swallows begin their journey south in mid-August. By mid-September, most swallows have disappeared.[1]
Studies using leg bands show that swallows breeding in Japan spend winter mainly in the southern parts of the Philippines, Indonesia, Malaysia, and Vietnam.[6] Another source lists southeastern China, central and northern Philippines, and Taiwan as common wintering spots, with the farthest record being central Indonesia.[1] Because different sources list different places, it is safer to read these locations with some flexibility, using words like "mainly" or "often." In Kagoshima Prefecture, it is also written that a group of about 1,000 swallows spends the winter there every year.[1] Swallows change their homes from eaves to water edges, reed beds, and southern countries throughout the year. But are Japanese swallows decreasing?
6. Are Swallow Numbers Really Decreasing in Japan?
In a 2012 national survey with 8,402 responses, 99% of respondents confirmed seeing swallows, and 86% confirmed seeing nests. No shrinking trend in distribution was observed, but 39% answered that swallows have decreased in the last 10 years.[4] This was a survey of impressions, not an actual count of individuals.
In Ishikawa and Toyama Prefectures, elementary school students have recorded nest numbers since 1972 and 1973. From the late 1990s, numbers decreased, but by around 2008, the count was about 15,000 nests, returning to the level of the 1970s.[1]
Most failures in raising chicks are due to natural events like attacks by crows or snakes, or nests falling. Removal by humans accounts for less than 10%.[3] In areas where the human population has declined due to depopulation, the risk of eggs or chicks being eaten may increase, potentially reducing nesting. The Wild Bird Society of Japan states that for breeding in cities, water edges, green spaces, and people who watch over swallows are important.[3]
7. What Can You Do While Waiting for Swallows to Return in Spring?
You can look for old nests on eaves or shop entrances near your home. If you find one, observe from a distance to see if it has a half-bowl shape, is attached to a wall, or if mud grains are visible. Do not touch the nest, do not enter private property or buildings, and stay within the range visible from the road.
Next spring, when swallows return, compare where they build nests and check if there are rice paddies or puddles nearby. The Wild Bird Society of Japan publishes a map of roosts and the "Good Night, Swallows" pamphlet. Since roosting entries happen in summer, use the map to find a nearby roost and compare its location to the nest you found.